Hi.I Just came upon a study test question concerning discontiguous networks.I've attached an Image.I understand the problem when there's , let's say , three routers In series and the middle router doesn't know to which of the edge routers It should route to when they have lans In the same classful network.
This question confused me a little because there's no router In the middle.
Ah I see.
I don't really understand why they can't ping eachother when they're directly connected.
Well they are not directly connected.
10.25.67.32/27 is directly connected only to A
10.25.67.64/27 is directly connected only to B
Do the routers reject the advertised network because there connected route to the same network has a lower administrative distance or a lower metric?
Well yes, they are rejected because of lower-higher AD comparison (directly connected AD is 0, EIGRP summary route AD is 5). Router A and also Router B thinks that the whole 10.0.0.0/8 resides on his side.
EIGRP automatically (because auto-summary is on) creates a discard route 10.0.0.0/8 Null0. Everything that will be destined to other that directly connected network from range 10.0.0.0/8 will be discarded thanks to this route. This is a problem with RIPv2 which is not that smart as EIGRP.
But as as Giuseppe have said regarding EIGRP and split-horizon rule.
If the other router accepts the route 10/8 it cannot advertise the SAME 10/8 out the same interface for the split horizon rule that says that a router will not advertise out an interface an IP prefix learned on that same interface.
As a result of this one router the router on the right has no return path for packets sourced from the IP subnet in 10/8 network and IP communication is broken.
IF aut-summary is disabled the two routers advertise two different IP subnets 10.25.67.32/27 and 10.25.67.64/32 and split horizon is not triggered anymore.
The same happens if the shared link has IP address in 10/8.
My problem Is I can understand the concept If there were a third router In between the two trying to ping from It's lan.It wouldn't know which router to choose.
Well the problem with 3 routers is kind of different. Mainly it leads to incorrect routing table on the middle router. Then the middle router can do load-balance and every second packet is sent to the incorrect router.